Nonwoven fabric is a cloth that is neither woven nor knitted. In fact, the nonwoven fabric is made by entangling randomly or directionally short fibers or long fibers (long filaments) to form a web structure and then bonded together mechanically, thermally, or chemically. In short, the nonwoven fabric is not based on yarn-by-yarn weaving or knitting, but is based on physical bonding of the fibers together. The nonwoven fabric represents a breakthrough in the conventional textile industry, and is characterized by short flow process, fast production rate, high output, low costs, wide adaptability, ease of acquirability of raw materials and the like.
The nonwoven fabric exhibits good filterability, permeability and absorption, and therefore is suitable to be used as a filter medium. For example, the nonwoven fabric may be used in the bag filters and the cartridge filters. However, conventional nonwoven fabrics are generally soft and require a supporting frame for mounting on the filter. FIG. 1 illustrates a cartridge filter of the prior art, comprising two circular end caps 12, 14, a net-shaped supporting frame 16 between the two end caps, and a filter medium made of the prior art nonwoven fabric mounted on an inner or outer side or both sides of the net-shaped supporting frame. This structure would increase the costs and is labor intensive in mounting and changing the filter because of the supporting frame; on the other hand, the performance of the filter medium may be diminished more or less because the net-shaped supporting frame can block the contact between the filter medium and the fluids to be treated, or cause deposit of dusts and/or pollutants. Thus, some efforts have been made in order to increase the stiffness or flexural stiffness of the nonwoven fabric.
Chinese patent application no. CN101678255A with the publication date of 24 Mar. 2010 has disclosed a long fiber nonwoven fabric made of synthetic fibers and a cylindrical bag filter made from the long fiber nonwoven fabric, wherein the long fiber nonwoven fabric is made of thermoplastic continuous filaments and is partially thermocompresson-bonded. The long fiber nonwoven fabric has a circular arc flexural stiffness per basis weight of 0.050 to 1.000 ((cN/2 cm)/(g/m2)), and an air permeation rate per basis weight of 0.010 to 0.500 ((cc/cm2/sec)/(g/m2)). The long fiber nonwoven fabric disclosed in this patent application is a spunbond nonwoven fabric, and produced from a method comprising the steps of melt-extruding a thermoplastic polymer through a spinneret, sucking and drawing the extrudate with an air sucker to from thermoplastic continuous filaments, allowing the filaments to be subjected to electrification opening and to be piled on a moving collection surface to form a fiber web, and allowing the fiber web to be subjected to pressure welding treatment with a flat roll and partially thermocompression-bonding the fiber web with hot embossing rolls to form the long-fiber nonwoven fabric. The obtained long fiber nonwoven fabric has good flexural stiffness, and is suitable to be used as the material of a bag filter since it exhibits excellent shape maintenance after backwashing air. However, this patent application indicates that the nonwoven fabric made of short fibers such as a conventional felt is poor in stiffness of the sheet and thus it is unsuitable. Although the long fiber nonwoven fabric disclosed in this patent application has a relative stiffness, the stiffness is enough to retain its shape only for the filters of small size (for example 30 cm) but the supporting frame is still required. For the filters of large size (for example 1 m or above), the long fiber nonwoven fabric is not capable of retaining its shape. In other words, such a nonwoven fabric is not self-sustaining.
A filter material of an air filter applied to a central ventilation system is disclosed in another Chinese patent application no. 101332385A. The filter material mainly comprises an upper layer of nonwoven fabric and a lower layer of nonwoven fabric as a substrate, and one or more composite layer(s) that is/are compounded between the upper and lower layers of nonwoven fabric. The filter material is characterized in that the composite layer is formed by sandwiching a layer of granular activated carbons between two layers of fiber cementing compound, wherein the layer of granular activated carbons consists of independent activated carbon granules spreading on one-by-one basis. The filter material is not only efficient and has low resistivity, but also maintains the adsorption performance of the activated carbons. Furthermore, the filter material maintains the hardness and stiffness required for the purpose of ventilation. Although this patent application does not mention which type the two layers of nonwoven fabric are, the filter material is composed by attaching the two nonwoven fabric layers to the intermediate composite layer, in such a manner that the hardness and the stiffness of the material are controlled by adjusting the amount of the activated carbons and the amount of the cementing compound to cater for the requirement of pleat processing. However, the filter material disclosed in this patent application is structurally complicated and not environmentally benign due to the use of cementing compound. In addition, the filtration performance of the filter material can be diminished when the activated carbons or some pores of the nonwoven fabric are blocked.
Moreover, currently available felt fabrics are texturally soft and pleats cannot be made thereon. Therefore, they cannot be used to produce a pleat-shaped filter in order for increased filtration area. The currently available felt fabrics do not have the mouldability, namely cannot be moulded into various shapes or configurations or cannot retain its shapes and configurations after it is moulded.
There has been no pleatable felt that has a stiffness sufficient for shape maintenance, exhibits excellent moldability, and is self-sustaining in the prior art. It has also not been found in the prior art that a pleat-shaped filter comprises a filter medium made of the felt, which is self-sustaining without the need of a supporting structure.